Thanks - what type of characters are allowed during sign-up that would be removed during "sanitization"?
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robarwebservicesFeb 18 '12 at 13:28

sanitize_title does the sanitizing. According to this it strips HTML & PHP tags. It seems to replace spaces with '-' somewhere. Note if you provide a nicename in wp_insert_user it doesn't do any sanitizing.
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Stephen HarrisFeb 18 '12 at 13:47